In situ synchrotron diffraction revealed a stepwise appearance of two new phases upon electrochemical lithium extraction from LiCoPO 4 . These phases were demonstrated to have the same olivine-like structure as the pristine compound. The lithium-deficient phases were proposed to be Li 0.7 CoPO 4 and CoPO 4 . The completely delithiated phase appears to be unstable in air and undergoes amorphization. The phase transitions are reversible, but a slow kinetics of the initial delithiation was identified by in situ synchrotron diffraction and the potentiostatic intermittent titration technique. We demonstrated that the electrochemical extraction of lithium is accompanied by significant electrolyte decomposition, contributing to the capacity loss upon cycling. The galvanostatic intermittent titration technique combined with impedance spectroscopy revealed self-discharge of the cell in the charged state. This study argues different mechanisms of lithium extraction from LiCoPO 4 in comparison with LiFePO 4 and LiMnPO 4 .

Sodium-ion batteries operating at ambient temperature hold great promise for use in grid energy storage owing to their significant cost advantages. However, challenges remain in the development of suitable electrode materials to enable long lifespan and high rate capability. Here we report a sodium super-ionic conductor structured electrode, sodium vanadium titanium phosphate, which delivers a high specific capacity of 147 mA h g−1 at a rate of 0.1 C and excellent capacity retentions at high rates. A symmetric sodium-ion full cell demonstrates a superior rate capability with a specific capacity of about 49 mA h g−1 at 20 C rate and ultralong lifetime over 10,000 cycles. Furthermore, in situ synchrotron diffraction and X-ray absorption spectroscopy measurement are carried out to unravel the underlying sodium storage mechanism and charge compensation behaviour. Our results suggest the potential application of symmetric batteries for electrochemical energy storage given the superior rate capability and long cycle life.

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